E. coli RNA polymerase contains a catalytic core of two ` subunits, one , and one subunit, with molecular masses of 36,512 Da, 150,619 Da, and 155,162 Da respectively. The core enzyme is a nonspecfic DNA binding protein that initiates transcription efficiently with little specificity. The holoenzyme has an additional regulatory subunit, most commonly e70, of molecular mass 70,236. By virtue of the e subunit, the holoenzyme recognizes only specific, duplex DNA sequences (promoters) and initiates transcription efficiently from these sites. The e70 subunit has been cloned and overexpressed in E. coli since the early 70's. Much effort has gone into its crystallization without success. We would like to undertake structural studies of e70 and of its fragments free and bound to DNA. We have begun by investigating the properties of e70 under conditions of limited proteolysis. Mass spectrometric analysis of the proteolytic fragments were used to define domains within e70 and also to determine proximate of components of the protein to RNA. A paper describing this work has been published.